Somatic DNA recombination plays key roles in the activation and diversification of antigen receptor genes. There are two different types of gene rearrangements in the immune system. One is V-(D)-J joining, which assembles the variable region gene segments of immunoglobulin (Ig) and T cell receptor genes. The other is class switch recombination, which replaces Ig heavy-chain constant region genes. This application aims to study the molecular mechanisms and regulation of somatic DNA recombination in lymphocytes. Specific aims are: 1) To identify cis-acting DNA elements that negatively regulate V-(D)-J joining; 2) To characterize the V-(D)-J recombinase and its genes; and 3) To determine the DNA region regulating the class-specificity of switch recombination. In aim 1, we will study what determines the cell-type specificity and the stage-specificity of V-(D)-J joining. We have recently found that the 3'Ckappa region may contain a cis-acting DNA element(s) that restricts Ig kappa gene rearrangement to the pre-B stage of B cell development. We will further characterize the regulatory element(s) and DNA binding proteins. Using recombination signal probes, we have isolated a CDNA clone coding for an HMG1-related DNA binding protein. Genetic backcross experiments have shown that the gene (rsb-1) is tightly linked to the recombination activating genes (rag-1 and rag-2) in mouse chromosome 2. In aim 2, we will study the possible roles of these genes in V-(D)-J joining. Unlike V-(D)-J recombination, class switching is an antigen-dependent process. Switch recombination is directed by some cytokines in a class- specific manner. In aim 3, we will identify DNA elements which determine the class-specificity of recombination. These studies will help our understanding of the molecular mechanisms of gene rearrangement in the context of lymphocyte development. They will also shed light on the basic processes of some immunodeficiencies and chromosome abnormalities caused by DNA arrangement.